VI THE EXPENDITURE OF ENERGY 281 



the vital food-stuffs at all, it appears desirable to dis- 

 tinguish the essential food-stuffs, or proteins, from the 

 accessory food-stuffs, or fats and carbohydrates— the 

 former alone being, in the nature of things, necessary to 

 life, while the latter, however important, are not abso- 

 lutely necessary. 



6. The Income and Expenditure of Energy.— It is 

 quite certain that nine-tenths of the dry, solid food which 

 is taken into the body, sooner or later leaves it in the 

 shape of carbonic acid, water, and urea ; and it is also 

 certain not only that the compounds which leave the 

 body are more highly oxidised than those which enter it, 

 but that all the oxygen taken into tlie blood by the lungs 

 is carried away out of the body in the various waste 

 products. 



The intermediate stages of this conversion are, however, 

 by no means so clear. It is highly probable that all the 

 food-stuffs which pass from the alimentary canal into the 

 blood, be they derived from proteins, or fats, or carbohy- 

 drates, become part and parcel of some tissue or other 

 (muscle, nervous tissue, glandular tissue, and the like), 

 before they are oxidised ; that indeed it is as constituent 

 elements of some tissue or other that they suffer oxidation, 

 and that the amount of oxidation going on in the blood is 

 very small. 



In the course of its oxidation, the food not only 

 supplies the energy which the body expends in doing work, 

 but also the energy which, as we have seen, the body loses 

 as heat. The oxidation of the food is indeed the ultimate 

 source of the heat of our bodies, all other causes being of 

 little moment. About this there can be no doubt, and it 

 is further probable that the oxidation which thus gives 

 rise to heat is not the oxidation of the elements of the 

 food as they are carried about in the blood, but the oxida- 

 tion of them in the tissues, more especially the muscles, 

 in the cells of which they may be pictured in close contact 

 with, and available for, the actual needs of the fabric. 



